The present invention concerns a polyolefin composition in powder form filled with hollow microspheres, adequate for the use in processes for the coating of metallic surfaces by way of flame-gun spraying.
Published European patent application No. 603 906 describes polyolefin compositions in powder form comprising a crystalline propylene homopolymer or a crystalline propylene random copolymer, an ethylene-propylene or ethylene/1-butene elastomeric polymer, polypropylene modified with polar groups, and optionally a nucleating agent and titanium dioxide.
Said compositions, having a melt index ranging from 15 to 150 g/10 minutes, are adequate for use in processes for the coating of metallic surfaces by using various techniques, among which is flame-gun spraying.
Spraying techniques using flame guns or sprayers allow one to obtain coatings with various degrees of thickness, and generally comprise two stages:
1) heating the metallic surface using a flame or other technique;
2) spraying the polyolefin composition powder while maintaining the flame lit.
However, penetration resistance of the coating obtained with the above mentioned compositions is not particularly high, and for some applications, such as the anticorrosion coating of underground or underwater pipes, it is inadequate.
On the other hand, it is known that the penetration resistance can be improved by adding to polyolefins the proper quantity of hollow microspheres, such as glass microspheres. In particular, published European patent application No. 473 215 describes the preparation of polyolefin compositions containing the above microspheres by treating said microspheres with a degradation agent of the polyolefin chain, and the subsequent mixing with the polyolefins.
The compositions thus obtained are used, among other things, for coating steel pipes by way of extrusion, or wrapping the pipes with a preformed tape.
Compared to the above mentioned technical solution, the polyolefin compositions in powder form of the present invention have the advantage of not requiring any treatment of the microspheres in order to obtain coatings with good physical-mechanical properties. Moreover, flame-gun spraying is easier to apply on the field compared with the extrusion coating techniques, for repairing sections of coating that were either damaged or removed during the application process for example.
Flame-gun spraying is also much less complex in terms of apparatus and operations when compared to the wrap coating with preformed tapes.
Published patent application WO 93/19927 describes an extrusion process for the preparation of insulating material comprising the following stages:
a) a first stage of where a thermoplastic resin, such as polypropylene for example, is fluidized;
b) a second stage where hollow microspheres are added under low shear stress conditions to the fluidized resin of the first stage.
The above mentioned insulating materials can be applied to the outside surface of pipes by way of extrusion or wrapping with ribbons of other preformed materials which are essentially made of the product obtained in stage b).
The extrusion coating technique is not described in detail in the above mentioned patent application, however, since the use of low shear stress in stage b) is necessary in order to avoid breaking the glass microspheres, it is inevitable that said procedure must also be followed during the extrusion coating.
In addition, as shown in published european patent application No. 473 215, coating pipes by way of extrusion is a process that normally requires high shear stress values, therefore said technique can be incompatible with the composition described in application WO 93/19927.
Moreover, patent application WO 93/19927 clearly states that extrusion coating requires the use of molds into which the melted polymer composition is conveyed.
In any case it is inevitable that the extrusion phase used for coating the pipes will cause a degradation of the properties of the material used for said coating, due to the additional breakage of the hollow microspheres and the further degradation of the melted polymer.
On the other hand, the polyolefin compositions in powder form of the present invention are applied to the metal surface simply by way of flame-gun spraying with no need for complicated apparatus, such as extrusion plants and molds where the molten material must be introduced, as well as particularly delicate operating conditions, such as having to use low shear stress. Moreover, applying the compositions of the present invention by way of flame-gun spraying allows one to obtain finished coatings having good physical-mechanical properties, as will be illustrated in the examples. On the other hand, the technical solutions described in the above mentioned documents of the prior art are limited to the preparation of insulating compositions having good physical-mechanical properties, leaving unresolved the problem of how to avoid the degradation of said properties following the application of the insulating compositions on the surface to be coated.
Considering the complexity of the technical solutions of the prior art, which leave unresolved the above mentioned problem, it is particularly surprising that using the coating compositions and process of the present invention one can obtain finished coating with good physical-mechanical properties without using complicated apparatus or having to take burdensome measures, such as pretreating the hollow microspheres.
Therefore, an object of the present invention is a polyolefin composition comprising (percentages by weight):
1) from 1% to 40%, preferably from 10% to 25%, of hollow microspheres;
2) from 60% to 99%, preferably from 75% to 90%, of a polyolefin composition comprising (percentages by weight):
a) from 25% to 95%, preferably from 50% to 90% of polypropylene homopolymer, polyethylene homopolymer, or a propylene/ethylene, propylene/C4-C10 xcex1-olefin, or propylene/ethylene/C4-C10 xcex1-olefin crystalline random copolymer, or mixtures thereof;
b) from 0% to 70%, preferably from 5% to 34%, of an ethylene-propylene or ethylene/C4-C10 xcex1-olefin elastomeric copolymer, optionally containing minor quantities of a diene;
c) from 0.5% to 10%, preferably from 2% to 6%, of polypropylene or polyethylene modified with polar groups in quantities ranging from 0.1% to 10%, of preferably from 0.2% to 5%;
d) from 0% to 10%, preferably from 0.6% to 10%, of titanium dioxide;
said polyolefin composition 2) having MFR (ASTM D-1238, condition L) from 2 to 150 g/10 min., preferably from 4 to 30, more preferably from 4 to 12 g/10 min., and being in powder form with not more than 3% of the granules having a diameter greater than 600 micrometers.